Electrode Position of Black Chromium Spectrally Selective Coatings From a Cr Ionic Liquid Solution

Thin Solid Films 519 (2011) 1845–1850

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r.com/ locate / ts f

Electrodeposition of black chromium spectrally selective coatings from a Cr(III)–ionicliquid solution

Sónia Eugénio a,⁎, Carmen Mireya Rangel a,b, Rui Vilar a, Ana Maria Botelho do Rego c

a Departamento de Engenharia de Materiais, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugalb Laboratório Nacional de Energia e Geologia, Paço do Lumiar 22, 1649-038 Lisboa, Portugalc Centro de Química-Física Molecular and IN, DEQB, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

⁎ Corresponding author.E-mail addresses: [email protected] (S. Eugénio), c

(C.M. Rangel), [email protected] (R. Vilar), amrego@ist.

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.10.029

a b s t r a c t

Article history:Received 20 January 2010Received in revised form 1 October 2010Accepted 7 October 2010Available online 21 October 2010

Keywords:Ionic liquidsElectrodepositionBlack chromium

In the present study, black chromium coatings were electrodeposited from a 1-butyl-3-methylimidazoliumtetrafluoroborate ([BMIm][BF4]) solution containing Cr(III) under potentiostatic control on copper substrates.The electrochemical behavior of the electrolyte was studied by cyclic voltammetry. The reduction of Cr(III)species in the solution is a two-step process, from Cr(III) to Cr(II) and from Cr(II) to Cr(0). The potential ofeach reaction shifts positively with the increase of the electrolyte temperature. The parameters for theelectrodeposition of black chromium films were optimized. Homogeneous black chromium films withspectrally selective optical properties were produced by applying a potential of −1.5 V for 1800 s, at anelectrolyte temperature of 85 °C. The coatings consist of a mixture of chromium oxide/hydroxide andmetallicchromium. They are amorphous and present a sub-micrometric granular structure.

[email protected] (A.M. Botelho do Rego).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Black chromium is widely used as a spectrally selective coatingmaterial in thermal energy conversion applications [1,2], due to itsoptical properties, high corrosion resistance and good thermalstability [3]. These coatings are usually obtained by electrodepositionfrom water-based hexavalent chromium (Cr(VI)) solutions [3], butthese electrolytes raise serious health and environmental concerns, soit is of the utmost importance to replace them as soon as possible.Ionic liquids have a number of unique properties that make themextremely interesting solvents for electrodeposition. They candissolve many organic and inorganic compounds, present a widerelectrochemical window and better chemical stability than water,negligible vapour pressure and high thermal and chemical stability. Awide range of metals, metal alloys and semiconductors has beenelectrodeposited from ionic liquid solutions [4,5], including reactivematerials such as aluminium, silicon, germanium and titanium [4–8],impossible to obtain by electrodeposition from aqueous solutions.Aluminium electrodeposition has been the object of particularattention and pure aluminium and several aluminium alloys havebeen electrodeposited from chloroaluminate-based ionic liquids [9–17] and from water and air-stable ionic liquid solutions [18]. Metals,such as silver [19], copper [20], nickel [21–23], zinc [21] andmanganese [21,24], which can also be obtained from aqueoussolutions, have frequently been electrodeposited from ionic liquid

electrolytes with better properties, due to the absence of hydrogenevolution.

The electrodeposition of chromium from ionic liquid-basedelectrolytes has been the object of only very few publications up tothismoment. Ali et al. [9] studied the electrodeposition of aluminium–

chromium alloys from CrCl2 solutions in AlCl3-N-(n-butylpyridiniumchloride). Depending on the applied current density, potential, andconcentration of the Cr(II) ion in the electrolyte, aluminium–

chromium alloys with chromium concentrations between 0 and to94 at.% were obtained. Abbott et al. [25–27] reported the electrode-position of chromium from an ionic liquid consisting of cholinechloride and hexahydrated chromium salt, producing a pale blue/greyamorphous film [25] or, if LiCl is added to the solution, a nanocrystal-line black chromium film [27]. In a more recent paper [26], the sameauthors describe the electrodeposition of hard and bright chromiumfilms from the same ionic liquid.

In the present paper, we report the electrodeposition of blackchromium thin films from an ionic liquid solution of Cr(III) ions. Theionic liquid used as solvent is 1-butyl-3-methylimidazolium tetra-fluoroborate ([BMIm][BF4]).

2. Experimental details

The preparation of the electrolyte and the electrochemicalexperiments were performed under an argon atmosphere in a glovebox with a gas purification system capable of ensuring that the H2Oand O2 contents in the atmosphere remained below 2 ppm. Solutionsof 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4])containing 0.4 M Cr(III) were prepared by adding CrCl3.6H2O to the

Fig. 1. Cyclic voltammogram of pure [BMIm][BF4] (dashed line) and [BMIm][BF4]–Cr(III) solution (solid line) at room temperature on a GC electrode. Scan rate: 50 mVs−1.

Fig. 2. Cyclic voltammograms obtained in the [BMIm][BF4]–Cr(III) solution at (a) 30 °C,(b) 50 °C and (c) 85 °C, on a GC electrode. Scan rate: 50 mVs−1.

1846 S. Eugénio et al. / Thin Solid Films 519 (2011) 1845–1850

ionic liquid and mixing both components for several hours undervacuum.

The electrochemical experiments were performed using anAutolab PGSTAT100 potentiostat controlled by the NOVA softwareand a conventional 3-electrode cell. Cyclic voltammograms wereobtained by sweeping the potential at a scan rate of 50 mVs−1 in thecathodic direction from the open-circuit potential up to the cathodiclimit, then reversing the scan until the anodic limit was reached. Atthis point, the scan direction was reversed again and the scan stoppedat the starting potential. A glassy carbon (GC) disc (A=0.071 cm2)was used as the working electrode. The counter-electrode was aplatinum plate (A=1 cm2) and the quasi-reference electrode (QRE) awire of the same material. All potentials are referred to the QRE. Thetemperature of the electrolyte was kept constant using a thermostat-ed bath and varied between 30 and 85 °C.

Electrodeposition was carried out on Cu plates, under potentio-static control. Prior to the experiments, the substrates were polishedwith 1000 grit SiC abrasive paper, cleaned in an alkaline solutionultrasonic bath for 15 min, immersed in a 1:1 HNO3:H2O solution for15 s, thoroughly rinsed with distilled water and dried with hot air.During the electrodeposition experiments, the electrolyte wasagitated using a magnetic stirrer to ensure a uniform supply ofmetal ions to the cathode–electrolyte interface and the temperaturewas kept constant by a thermostated bath. After deposition, thesamples were rinsed with deionized water and acetone and driedwith hot air.

The coatings were characterized by scanning electron microscopy(SEM) and energy dispersive X-ray spectrometry (EDS) using a JEOLmodel 7001F field-emission gun, a scanning electronmicroscope withan Oxford X-ray EDS spectrometer, with an acceleration voltage of15 kV. The chemical composition of the films was studied by X-rayphotoelectron spectroscopy (XPS) using a XSAM800 (KRATOS)spectrometer equipped with an AlKα source (photon energy equalto 1486.6 eV). The XPS spectra were submitted to a Shirley-typebackground subtraction and fitted with Gaussian–Lorentzian productfunctions using the XPSPeak 4.1 software. The crystallographicstructure was assessed by glancing-incidence X-ray diffraction (GI-XRD) analysis performed using a Siemens D5000 diffractometer, withCu Kα radiation and an incidence angle of 1°. The peaks observed inthe diffractograms were indexed by comparison with JCPDS databasefiles. Transmission electron microscopy (TEM) was performed using aHitachi 8100 microscope, at an operation voltage of 200 kV. Thesamples for TEM were obtained by scratching the coatings from thesubstrate and collecting the coating fragments on a 200 mesh coppergrid coated with a formvar film. The optical properties of the blackchromium coatings were evaluated by spectral reflectance measure-ments in the near and medium infrared range. The spectra wereobtained with a Nicolet Fourier-transform IR spectrometer, equippedwith a reflection accessory.

3. Results

The cyclic voltammograms of pure [BMIm][BF4] and of a solution ofthis ionic liquid containing 0.4 M Cr(III), on a GC electrode arepresented in Fig. 1. Pure [BMIm][BF4] is electrochemically stable in therange −2.5 to +1 V, in agreement with published studies on theelectrochemical behavior of this liquid [28]. The presence of Cr(III)ions in the solution changes significantly the voltammogram, whichshows new cathodic and anodic peaks related to the reduction andoxidation of the Cr species in the solution. In the forward scan, a firstcathodic peak is detected at −0.9 V (IC), corresponding to thereduction of Cr(III) to Cr(II) in the solution. A second cathodic peak,appearing at −1.6 V (IIC), corresponds to the reduction of Cr(II) to Cr(0). The ratio between the integrated charges of peaks IC and IIC islarger than 1, suggesting that only part of the Cr(II) is reduced to Cr(0). In the reverse scan, a first anodic peak occurs at −1 V (IA), which

can be attributed to the oxidation/stripping of Cr(0) formed duringthe forward scan. Again, the ratio between the integrated charges ofthe cathodic (IIC) and of anodic peaks (IIA) of the reaction is higherthan 1, indicating that this oxidation is only partial. A second peak isobserved at−0.2 V (IIA), which corresponds to the oxidation of the Cr(II) formed in the forward scan that was not reduced to Cr(0). Thepeakmay also include a contribution from the oxidation of the speciesformed in the reaction corresponding to peak IIA, as confirmed by therelation between the peak integrated charges.

The effect of the temperature on the electrochemical behavior ofthe 0.4 M Cr(III)–[BMIm][BF4] solution is illustrated in Fig. 2.Increasing the electrolyte temperature leads to an increase in thecurrent density and to a shift of the reduction peaks to less negativepotentials, especially for temperatures in the range 50 to 85 °C. Thisevolution can be attributed to the decrease of the solution viscosity,which enhances the mass transport of the active species in thesolution and, hence, the current density. This effect is frequentlyobserved in ionic liquid-based electrolytes [19].

The electrodeposition experiments were carried at 30, 50 or 85 °Cwith applied potentials varying between −1.5 and −2 V anddeposition times of 900 and 1800 s. Several combinations of theseparameters were tested in order to uncover a set of experimentalconditions allowing the deposition of homogeneous and adherentfilms. Fig. 3 illustrates the influence of the electrolyte temperature anddeposition time on the surface topography of the coatings obtained ata potential of−1.5 V. For an electrolyte temperature of 30 °C the filmspresent a brownish color and are iridescent, while for 50 and 85 °Cthey are uniformly black. The coatings are formed of globularaggregates of nanometric particles (Fig. 4). The size of these aggregatesincreases with increasing electrolyte temperature and deposition time.

Fig. 3. SEM images of the (a) copper substrate surface, and black chromium films produced by potentiostatic deposition at −1.5 V for 900 s at (b) 30, (c) 50 and (d) 85 °C and for1800 s at (e) 30, (f) 50 and (g) 85 °C.

1847S. Eugénio et al. / Thin Solid Films 519 (2011) 1845–1850

The surface topography of the coatings is strongly related to the initialsurface topography of the substrate, and reveals themicrostructure of thesubstratematerial. Prior to the electrodeposition experiments, the coppersubstrates were etched in diluted nitric acid and present a surface reliefrevealing grain boundaries and other microstructural features (Fig. 3a).This topographic relief leads to local current density maxima where thenucleation and growth of the coating material are faster, resulting in anaccentuation of the initial surface relief. The small thickness of the coatingalso contributes to this effect because it is not sufficient for a completeleveling of the film surface.

A similar effect of the electrolyte temperature and deposition timeis observed when the deposition is carried out at −1.6 V, except forfilms deposited during 1800 s at 85 °C, which show a cauliflower-likemorphology, typical of an excessive cathode polarization. For morecathodic potentials (−1.8 and −2.0 V), the films became discontin-uous independently of the electrolyte temperature and depositiontime. Some films formed at these potentials and at 85 °C showevidence of gas evolution during the electrodeposition process.

Cross-sections of a black chromium film electrodeposited at−1.5 V, for 1800 s at a temperature of 85 °C are presented in Fig. 5.The film thickness is approximately 2 μm and the cross-sectionmicrostructure structure appears to be quite featureless.

Chemical analysis of the deposited films, performed by EDS,identified chromium and oxygen as themain elements constituting allthe films. The chemical composition does not depend on theelectrodeposition parameters. The penetration of the electron probein the material being larger than the thickness of the films, peaksrelated to the copper substrate are observed in the EDS spectra. Thefilms shown in Fig. 3c, d, f and g present only very small copper peaksin the EDS spectra confirming a larger film thickness. Small quantitiesof Cl and F are also detected, suggesting that some electrolyte istrapped in the film during its formation.

XPS was performed on the films without any preliminarysputtering cleaning. The XPS spectrum (Fig. 6) presents peakscorresponding to Cr, O and F, in agreement with the EDS results, aswell as the peaks of C and N. The Cr 2p3/2 peak (Fig. 6) can be peakfitted with three peaks. The peak centered at 574.2±0.2 eV isassigned to metallic Cr (Cr(0)) [29,30]. The peak at 576.7±0.2 eVcan be attributed to Cr(III) in Cr2O3 [29], Finally, a peak centered at577.8±0.2 eV is assignable to Cr(OH)3 since the values found in theliterature for the hydroxide, when presented together with the oxide,are systematically higher than the ones for the oxide by around 1 eV[31,32]. The O1s spectrum (Fig. 6) has been peak fitted with threepeaks. The low energy peak at 530.1 eV is attributed to O2− in Cr2O3

Fig. 4. Detailed morphology of the black chromium film produced by potentiostaticdeposition at −1.5 V for 1800 s at 85 °C.

Fig. 5. Cross-sections of the black chromium film obtained from potentiostaticdeposition at −1.5 V for 1800 s, at 85 °C on the copper substrate.

1848 S. Eugénio et al. / Thin Solid Films 519 (2011) 1845–1850

[30] while the high energy peak at 533.2 eV is attributed to adsorbedwater [29]. The intense peak at 531.5 eV can be attributed tohydroxide/oxyhydroxide species and adsorbed oxygen [30,33].These results indicate the presence of both Cr2O3 and Cr(OH)3 inthe most external layers of the film. The peak of carbon C1s can bepeak fitted with 3 peaks (Fig. 6). The peak of carbon C1s can bedeconvoluted in 3 peaks (Fig. 6). The peaks at 286.9 and 288.6 eV canbe attributed to the carbon of the imidazolium ring of the [BMIm][BF4]ionic liquid [34], while the peak at 285 eV is typical of contaminationof the surface by hydrocarbons. Fluorine and nitrogen peaks mayoriginate from the traces of the [BMIm][BF4] electrolyte [34] adsorbedat the surface of the coatings.

Diffractograms of the coatings are presented in Fig. 7. All peaks inthe diffractograms can be ascribed to the copper substrate. Thedifference in the relative intensity of diffraction peaks in thediffractograms is due to the crystallographic texture of the polycrys-talline copper substrates that were obtained from rolled copperplates. No peaks related tometallic chromium or chromium oxides aredetected.

Fig. 8a shows a TEM image of a thin film produced byelectrodeposition at 85 °C, with an applied potential of −1.5 V for1800 s. The thin film presents the typical mottled contrast of anamorphous structure. The amorphous nature of the film is confirmedby the selected area electron diffraction pattern (Fig. 8b), whichshows only diffuse halo-like rings. A similar behavior is observed in allfilms, independent of the deposition parameters.

The spectral reflectance of a coating obtained by potentiostaticdeposition at −1.5 V during 900 s is presented in Fig. 9. The values ofα and ε were calculated using the method described by Duffie andBeckman [2]. The coating presents a high absorbance (α=0.97) forradiation with wavelengths between 0.8 and 4 μm (near-IR region)and a low emittance (ε=0.19) for higher wavelengths (mid-IR

region).These values confirm the potential applications of thesecoatings for solar energy applications.

4. Discussion

The present results show that a 0.4 M Cr(III)–[BMIm][BF4] solutioncan be used as an electrolyte for the electrodeposition of blackchromium coatings on copper substrates. The reduction of Cr(III) inthe Cr(III)–[BMIm][BF4] ionic liquid solution is a two-step processinvolving two consecutive reduction reactions, the first one from Cr(III) to Cr(II) and from Cr(II) to Cr(0) (Fig. 1). These results are similarto those obtained with aqueous [35,36] and choline chlorideelectrolytes [25,27] containing Cr(III). However, contrary to whatwas observed for the choline chloride-based electrolytes [25,27], inthe present system the electrode surface is not passivated after thereduction of Cr(III) to Cr(II), suggesting that the Cr(II) exists in thesolution.

Homogenous and adherent black chromium films have beenelectrodeposited on copper substrates at a constant potential of−1.5 V. The thickness of the films (0.5–2 μm) is in the optimum rangefor solar energy applications [37,38] and they present good opticalproperties, similar to black chromium coatings electrodeposited fromCr(VI) aqueous solutions [38].

The films are formed of globular clusters of nanometric particles(Fig. 3) similar to those observed in black chromium films depositedfrom Cr(VI) [37–42] aqueous solutions. Frequently the chromiumconcentration in such films varies as a function of the distance to thesubstrate, the region near the substrate being richer in metallicchromium and the surface of the film richer in Cr2O3 and Crhydroxides [40,43]. In the present study, the small amount of metallicchromium detected by XPS (Fig. 5) at the surface of the films can also

Fig. 6. XPS spectra of black Cr film obtained from potentiostatic deposition at −1.5 V for 1800 s.

1849S. Eugénio et al. / Thin Solid Films 519 (2011) 1845–1850

indicate the presence of a Cr concentration gradient throughout thethickness of the film. Chromium films obtained from choline chloride-based solutions containing LiCl [27] are also black in color but theyseem to consist only of metallic chromium.

The structure of the films could not be fully characterized. NeitherSEM (Fig. 3) nor TEM (Fig. 7) analysis allowed observing metallicchromium and chromium oxide/hydroxide particles. The microstruc-ture of black chromium coatings electrodeposited from aqueous Cr(VI) solutions is quite complex and has been the subject of severalstudies. Inal et al. [40] observed that the films consisted of crystals ofCr and Cr2O3 and of Cr aggregates engulfed in Cr2O3 while Mabon et al.[41] studied the black chromium films by TEM concluding that Cr is

Fig. 7. Diffractograms of black Cr films deposited on copper substrates by potentiostaticdeposition at −1.5 V for 900 s (a) and 1800 s (b).

Fig. 8. (a) TEM image of a coating electrodeposited at 85 °C, with an applied potential of−1.5 V for 1800 s. (b) Selected area electron diffraction pattern.

Fig. 9. Optical properties of the black Cr film deposited on copper substrates bypotentiostatic deposition at −1.5 V for 1800 s.

1850 S. Eugénio et al. / Thin Solid Films 519 (2011) 1845–1850

encapsulated in an amorphous Cr2O3 matrix. Sweet et al. [38] usedeffective medium theory models for the dielectric function of acomposite system of Cr metal, Cr2O3 and void volume and concludedthat the best overall agreement between theory and experiments wasthe Maxwell–Garnett dielectric function for Cr spheres surrounded byCr2O3 distributed in an air matrix. In black chromium films obtainedfrom [BMIm][BF4]–Cr(III) solutions no crystalline chromium phaseswere detected by XRD, which indicates that the microstructure of thefilms can be different.

5. Conclusions

Black chromium electrodeposition from a [BMIm][BF4]–Cr(III)solution is reported. The electrodeposition of black chromium occursin two steps involving Cr(III) and Cr(II) electroactive species. The filmsare amorphous, present a nodular nanostructured morphology andare formed of chromium oxide, chromium hydroxide and metallicchromium. The black chromium films are also found to be spectrallyselective.

Acknowledgements

This work was financially supported by the European Communityunder the project IOLISURF (STRP-517002) and Fundação para aCiência e Tecnologia (FCT) under the project PTDC/CTM/68847/2006.Sónia Eugénio acknowledges a PhD grant from FCT.

References

[1] W.F. Bogaerts, C.M. Lampert, J. Mater. Sci. 18 (1983) 2847.[2] J.A. Duffie, W.A. Beckman, Solar Engineering of Thermal Processes, JohnWiley and

Sons, New York, 1980.[3] M. Schlesinger, M. Paunovic (Eds.), Modern Electroplating, John Wiley & Sons,

Inc., New York, 2000.[4] F. Endres, D.R. MacFarlane, A.P. Abbott (Eds.), Electrodeposition from Ionic

Liquids, Wiley-VCH, 2008.[5] F. Endres, Chemphyschem 3 (2002) 144.[6] S.Z.E. Abedin, N. Borissenko, F. Endres, Electrochem. Commun. 6 (2004) 510.[7] F. Endres, M. Bukowski, R. Hempelmann, H. Natter, Angew. Chem. Int. Ed. 42

(2003) 3428.[8] I. Mukhopadhyay, C.L. Aravinda, D. Borissov, W. Freyland, Electrochim. Acta 50

(2005) 1275.[9] M.R. Ali, A. Nishikata, T. Tsuru, Electrochim. Acta 42 (1997) 2347.

[10] M.R. Ali, A. Nishikata, T. Tsuru, J. Electroanal. Chem. 513 (2001) 111.[11] S. Caporali, A. Fossati, A. Lavacchi, I. Perissi, A. Tolstogouzov, U. Bardi, Corros. Sci.

50 (2008) 534.[12] R.T. Carlin, P.C. Trulove, H.C.D. Long, J. Electrochem. Soc. 143 (1996) 2747.[13] J.C. Li, S.H. Nan, Q. Jiang, Surf. Coat. Technol. 106 (1998) 135.[14] Q. Liao, W.R. Pitner, G. Stewart, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 144

(1997).[15] Q.X. Liu, S.Z. El Abedin, F. Endres, Surf. Coat. Technol. 201 (2006) 1352.[16] W.R. Pitner, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 143 (1996) 130.[17] Y.G. Zhao, T.J. VanderNoot, Electrochim. Acta 42 (1997) 3.[18] S.Z. El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Chem-

physchem 7 (2006) 1535.[19] R. Bomparola, S. Caporali, A. Lavacchi, U. Bardi, Surf. Coat. Technol. 201 (2007)

9485.[20] S.Z. El Abedin, A.Y. Saad, H.K. Farag, N. Borisenko, Q.X. Liu, F. Endres, Electrochim.

Acta 52 (2007) 2746.[21] M.J. Deng, P.Y. Chen, T.I. Leong, I.W. Sun, J.K. Chang, W.T. Tsai, Electrochem.

Commun. 10 (2008) 213.[22] W. Freyland, C.A. Zell, S.Z.E. Abedin, F. Endres, Electrochim. Acta 48 (2003) 3053.[23] M.J. Deng, I.W. Sun, P.Y. Chen, J.K. Chang, W.T. Tsai, Electrochim. Acta 53 (2008)

5812.[24] P.Y. Chen, C.L. Hussey, Electrochim. Acta 52 (2007) 1857.[25] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, Chem. Eur. J. 10 (2004) 3769.[26] A.P. Abbott, K.S. Ryder, U. Konig, Trans. Inst. Met. Finish. 86 (2008) 196.[27] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, J. Archer, C. John, Trans. Inst. Met.

Finish. 82 (2004) 14.[28] A.M. O'Mahony, D.S. Silvester, L. Aldous, C. Hardacre, R.G. Compton, J. Chem. Eng.

Data 53 (2008) 2884.[29] NIST X-ray Photoelectron Spectroscopy Database: http://srdata.nist.gov/xps/.[30] V. Maurice, W.P. Yang, P. Marcus, J. Electrochem. Soc. 141 (1994) 3016.[31] A. Ithurbide, I. Frateur, A. Galtayries, P. Marcus, Electrochim. Acta 53 (2007) 1336.[32] D. Shuttleworth, J. Phys. Chem. 84 (1980) 1629.[33] M.F. Al-Kuhaili, S.M.A. Durrani, Opt. Mater. 29 (2007) 709.[34] S. Caporali, U. Bardi, A. Lavacchi, J. Electron. Spectrosc. 151 (2006) 4.[35] F.I. Danilov, V.S. Protsenko, T.E. Butyrina, Russ. J. Electrochem. 37 (2001) 704.[36] Y.B. Song, D.T. Chin, Electrochim. Acta 48 (2002) 349.[37] C.M. Lampert, J. Washburn, Sol. Energ. Mater. 1 (1979) 81.[38] J.N. Sweet, R.B. Pettit, M.B. Chamberlain, Sol. Energ. Mater. 10 (1984) 251.[39] G. Zajac, G.B. Smith, A. Ignatiev, J. Appl. Phys. 51 (1980) 5544.[40] O.T. Inal, M. Valayapetre, L.E. Murr, A.E. Torma, Sol. Energ. Mater. 4 (1981) 333.[41] J.C. Mabon, O.T. Inal, A.J. Singh, Sol. Energ. Mater. 7 (1982) 359.[42] P.M. Driver, P.G. McCormick, Sol. Energ. Mater. 6 (1982) 159.[43] S. Surviliene, L. Orlovskaja, S. Biallozor, Surf. Coat. Technol. 122 (1999) 235.